The body, however, is clever. Expose it to a given stimulus enough and it will adapt out of necessity. Joshua Guy and his colleagues published an article in the journal Sports Medicine (March 2015) looking at this very topic and found that after 8 to 14 days of heat exposure athletes exercised with lower heart rates than before at the same speed, sweated earlier and had lower core temperatures. These adaptations to heat training made the athletes more efficient in all conditions.
This seemed to confirm the findings of Santiago Lorenzo’s groundbreaking 2010 study. The then-University of Oregon professor found that after a 10-day exercise protocol in the heat (100 degrees), the heat-trained athletes increased their VO2 max and power output by 8 percent and 5 percent, respectively, in hot conditions. The bigger surprise was their VO2 max, time trial performance and power output at lactate threshold were also all 5-6 percent higher than before. Plasma volume and maximal cardiac output were also up 6-9 percent for the heat trained group. The control group training in 55-degree weather showed no significant gains in any of these categories.
Implementing Heat Training
It’s easy to see journal articles touting the efficacy of heat training and want to jump in immediately. Before we do, I want to point out several things from these articles:
The cyclists in Lorenzo’s study were well-trained athletes exercising in a controlled environment. Though the temperature was very high in the heat chamber, the humidity was quite low (30 percent).
There were only 20 athletes in Santiago’s study. Other studies show similar constraints.
Guy’s paper was a review paper, and more research needs to be done to see if these gains translate directly to runners in a sustainable manner and how long they last.
There are also tradeoffs inherent in heat training. No matter how well you adapt, you’ll always run a 5K slower in 90-degree heat versus 50-degree weather. The same goes for intervals or tempos. While sprinters may find running in warm conditions advantageous, distance runners suffer sharp declines as the duration increases.
For those reasons, I recommend the following as the best way to implement heat training into your own program.
Start with only easy runs in the heat. To see how well you’ll tolerate extreme summer training, run shorter, easier runs during the heat of day, and save workouts for the cooler mornings or less-humid evenings.
When you implement workouts, start with shorter intervals. One of the reasons we find ourselves wanting to stop or slow down during summer training is because it’s easier to cool the body when it’s not generating additional heat. By running sprints and shorter intervals with longer recoveries, you’ll give yourself some literal chill time to assess whether you’re acclimating properly to the stress of heat and humidity.
Run in areas that have water fountains or carry a handheld water bottle. If you have the option, bring the electrolytes! Dehydration is a guarantee during the hotter months, but total water lost is less critical than intracellular hydration, which can be maintained by ingesting small to moderate amounts of fluids during exercise. (This is most evident in big city marathons, where elites may lose 3-6 percent of their bodyweight from dehydration but not slow down). If you have the option to mix electrolytes with your water, all the better. Salt, magnesium, and potassium help get water into your cells and replace what’s lost during sweating.
I can’t say definitively where spring went. I’m pretty certain we went from an endless winter to an extreme summer in the blink of an eye. But the silver lining is there. Use caution and discretion implementing it and you, too, may find yourself running the best times of your life this fall after a much more enjoyable June, July and August.
By Philip Latter. Latter is a former senior writer at Running Times and co-author of Running Flow and Faster Road Racing. His work has also appeared in Runner's World, runnersworld.com, and ESPN.com. He currently coaches athletes at The Running Syndicate, in addition to his day job coaching high school runners at Brevard High School (NC).